Libro.fm's books are all mp3 and DRM-free.
I have a subscription, and their subscription service is also great. They give you credits every month, but they don't expire.
That means some months I don't buy anything, and then one month I buy three books, all "for free".
A very small number of books are not available from credits due to publisher restrictions, and on those Libro.fm offers heavy discounts (usually around 40%) to subscribers.